1. Field of the Invention
The present invention relates to a secondary battery and, more particularly, to a secondary battery that can improve the sealing property of an electrolytic solution injection hole.
2. Description of the Prior Art
Generally, a secondary battery is a battery which can be repeatedly charged and discharged, differently from a disposable battery which is not rechargeable. The secondary battery has been widely used for advanced electronic devices such as cellular phones, portable computers and camcorders. A lithium secondary battery has a 3.6V operating voltage, which is three times greater than that of a nickel-cadmium battery or a nickel-hydrogen battery, and it has a higher energy density per unit weight compared to other types of batteries. Thus, the lithium secondary battery has been rapidly developed in recent years.
The lithium secondary battery uses lithium group oxides as a cathode active material, and carbonic material as an anode active material. Generally, lithium secondary batteries are divided into liquid electrolyte batteries and polymer electrolyte batteries according to the kind of electrolyte used. A battery using a liquid electrolyte is called a lithium ion battery and a battery using a polymer electrolyte is called a lithium polymer battery. The lithium secondary battery is manufactured in various types, typically, cylindrical, angular and pouch types.
Generally, an electrolytic solution injection hole is formed at a cap plate of the secondary battery for injecting a liquid electrolyte into the secondary battery. A stopper is seated on the electrolytic solution injection hole, and the electrolytic solution injection hole is sealed by pressing the stopper.
However, the electrolytic solution leaks when a gap is generated between an outer circumferential surface of the stopper pressed in the electrolytic solution injection hole and an inner circumferential surface of the electrolytic solution injection hole. The leaked electrolytic solution causes sparks at the time of welding of an outer surface of the stopper, and simultaneously generates dregs, such as black dirt. Thus, a pin-hole is formed between inner circumferential surfaces of the stopper and electrolytic solution injection hole, thereby lowering the sealing property of the electrolytic solution injection hole prominently.